For many internal combustion engine applications, such as for powering heavy trucks, it is desirable to operate the engine in a braking mode. This approach involves converting the engine into a compressor by cutting off the fuel flow and opening the exhaust valve for each cylinder near the end of the compression stroke.
An early technique for accomplishing the braking effect is disclosed in U.S. Pat. No. 3,220,392 to Cummins, wherein a slave hydraulic piston located over an exhaust valve opens the exhaust valve near the end of the compression stroke of an engine piston with which the exhaust valve is associated. To place the engine into braking mode, three-way solenoids are energized which cause pressurized lubricating oil to flow through a control valve, creating a hydraulic link between a master piston and a slave piston. The master piston is displaced inward by an engine element (such as a fuel injector actuating mechanism) periodically in timed relationship with the compression stroke of the engine which in turn actuates a slave piston through hydraulic force to open the exhaust valves. The compression brake system as originally disclosed in the '392 patent has evolved in many aspects, including improvements in the control valves (see U.S. Pat. Nos. 5,386,809 to Reedy et al. and U.S. Pat. No. 4,996,957 to Meistrick) and the piston actuation assembly (see U.S. Pat. No. 4,475,500 to Bostelman). A typical modern compression braking system found in the prior art is shown in U.S. Pat. No. 4,423,712 to Mayne et al. where the exhaust valves are normally operated during the engine's power mode by an exhaust rocker lever. To operate the engine in a braking mode, a control valve separates the braking system into a high pressure circuit and a low pressure circuit using a check valve which prevents flow of high pressure fluid back into the low pressure supply circuit, thereby allowing the formation of a hydraulic link in the high pressure circuit. A three-way solenoid valve, positioned upstream of the control valve, controls the flow of low pressure fluid to the control valve, and thus, controls the beginning and end of the braking mode.
The system disclosed in Mayne el al. also includes a reset valve which operates to cause the slave piston to retract after an initial opening of the exhaust valve during braking. As a result, the exhaust valve is closed prior to the end of the expansion stroke and before the hydraulic pressure drops due to a return motion of the master piston. This design advantageously avoids shock or asymmetric loading of the crosshead by the exhaust rocker arm at the start of the main opening event of the exhaust valve following the initial opening event. However, the reset valve is formed in the slave cylinder for contact, and thus tripping, by the slave piston. Thus, the reset valve relies on the movement of the slave piston relative to the piston housing. Also, the reset valve is closed when the engine is operating in a power mode thereby undesirably creating a small volume in the slave piston which is not connected to the low pressure drain. As a result, air pockets may form in this volume disrupting slave piston or reset valve motion thereby possibly adversely affecting the predictability of the braking event.
U.S. Pat. No. 5,680,841 to Hu discloses an electro-hydraulic engine valve control system for permitting engine braking operation which includes a slave piston mounted in a bore formed in a rocker lever, a control oil circuit formed in the rocker lever and rocker shaft and a check valve positioned in the oil control circuit between the slave piston and a central oil passage formed in the rocker shaft. The system also includes an electronically controlled valve and an accumulator positioned along the oil control circuit. However, this system uses a cam profile which causes the exhaust valve to completely close between the initial opening of the exhaust valve and the primary opening of the exhaust valve during braking. This invention also requires the electronic control solenoid valve to open and close every engine cycle in both power and braking modes. Also, this design appears to undesirably require a solenoid for each cylinder.
Therefore, there is a need for an improved engine compression braking system having an integral rocker lever and reset valve capable of effectively avoiding asymmetric loading of a valve crosshead while providing accurate and predictable compression braking.